The present invention relates to the converters for three-phase electric power, such as the PWM inverter or the cycloconverter.
A treatise on the decoupling control of a three-phase converter, "ON DECOUPLING CONTROL INSTANTANEOUS REACTIVE POWER CONTROL BY PWM-CONTROLLED POWER CONVERTER" appears in the record of the Semiconductor Power Conversion Conference, the Institute of Electrical Engineer of Japan, 1984 (SPC-84-80). This prior art will be explained with reference to FIG. 5 taken from Diagram 6 on page 54 of the above paper but using the same symbols as in the present invention.
In preparation for the explanation, reference will be had to FIG. 6 in regard to the arrangement of the current control system of a PWM (pulse width modulation) inverter being a representative three-phase converter to which the present invention and the prior art are directed. Referring to the figure, an inverter 1 includes six switches S.sub.1 thru S.sub.6 each of which is configured of a transistor and a diode connected in inverse parallel relationship to the transistor. The outputs of the inverter 1 are connected to an A.C. power source 5 through reactors 2, 3 and 4 each of which has an inductance L.sub.S.
The inverter 1 is a so-called high-frequency PWM inverter in which the instantaneous values of output currents are controlled by high-frequency switching based on triangular waves comparison at, for example, 1 kHz, and it converts electric power bidirectionally between a D.C. power source 20 and the A.C. power source 5. Here, the A.C. power source 5 is not always a commercial power supply or the like, but it generally expresses the counter electromotive force of an induction machine, the voltage of the capacitor of a filter, etc. as a voltage source. The output currents I.sub.AU, I.sub.AV and I.sub.AW of the inverter 1 are respectively detected by current sensors 6, 7 and 8, and they are transformed into values I.sub.Ad and I.sub.Aq on d - q coordinates by a coordinate transformation circuit 9, these values being fed back.
A current control block 12 detects the differences of the values I.sub.Ad and I.sub.Aq from current commands I.sub.Aq * and I.sub.Aq * by means of subtraction circuits 10 and 11, respectively, and applies them to a current-control amplifier 12-a. The current control block 12 produces the voltage commands V.sub.Ad * and V.sub.Aq * on the d - q coordinates generated by the inverter 1, and it applies them to a coordinate transformation circuit 13. The coordinate transformation circuit 13 transforms the voltage commands V.sub.Ad * and V.sub.Aq * from the d - q coordinates into the U-V-W coordinates to produce three-phase voltage commands V.sub.AU *, V.sub.AV * and V.sub.AW *, to be applied to the PWM circuit 14. The PWM circuit 14 compares a triangular wave with each signal, for example, the signal V.sub.AU * as illustrated in FIG. 7 by way of example, and applies switching signals to isolation amplifiers A.sub.1 and A.sub.2 to turn "on" the switch S.sub.1 and "off" the switch S.sub.2 of the corresponding phase, namely, the U-phase when the signal V.sub.AU * is greater than the triangular wave, whereby the on/off states of the transistors S.sub.1 and S.sub.2 are controlled. As a result, the average value of the output voltage V.sub.AU of the U-phase in the pertinent cycle of the triangular wave becomes a value corresponding to the voltage command V.sub.AU * of the U-phase. The on/off control of the transistor switches as stated above are repeated for all the switches S.sub. 1 -S.sub.6 every cycle of the triangular wave signal of the PWM, whereby the currents of the respective phases flowing through the reactors 2-4 are instantaneously controlled in accordance with the current commands I.sub.Ad * and I.sub.Aq *. With such a high-frequency PWM as a premise, the current control in FIG. 5 based on the known material operates as stated below.
A control circuit in FIG. 5 produces signals in which the differences between the current command I.sub.Ad *, I.sub.Aq * and current feedback signal I.sub.Ad, I.sub.Aq are respectively amplified by a gain K. Further, it produces signals in which the respective feedback signals I.sub.Ad and I.sub.Aq of the d- and q-axes are multiplied by the output angular frequency .omega. and output inductance L.sub.S of the converter. The q-axis signal of these signals is subtracted from the inverter voltage command of the d-axis, and the d-axis signal is added to the inverter voltage command of the q-axis, whereby the current components of the d- and q-axes are prevented from interfering with each other.
The prior-art system thus far explained has had the following problems:
(a) Since the current control circuit is arranged on the d - q-axis coordinates to implement the decoupling control, the detected current signals must be transformed into signals on the d - q coordinates and the current control outputs must be transformed into the three-phase coordinates again. PA1 (b) The decoupling method of the three-phase inverter in the paper SPC-84-80 of the conference record of the Research of Semiconductor Power Conversion of JIEE deals with the decoupling control in a continuous time system. A decoupling control in a sampled data system is performed under the assumption that the sampling time of the system is sufficiently short. That is, the prior-art inverter is difficult to adapt to sampling when the sampling time is not short enough. PA1 (1) First, the formula of discrete-time control system with decoupling control on d - q coordinates is derived using a matrix. PA1 (2) Subsequently, the formula is transformed from the d - q coordinates into the U-V-W coordinates, to obtain a matrix formula for decoupling the discrete-time control system on the U-V-W coordinates. PA1 (3) Finally, a practicable control is set up on the basis of the matrix.
An object of the present invention is to provide a method of decoupling control for a three-phase converter such as inverter which dispenses with the d - q coordinate transformation.
A further object of the present invention is to provide a method of decoupling control in a sampled-data system, which achieves precise decoupling control even in case of a long sampling time, thereby to realize an excellent control performance by a microprocessor, a digital signal processor, or the like.
In order to accomplish the objects, according to the present invention, a discrete-time control system with decoupling control is constructed on U-V-W coordinates in the following way:
The decoupling current control system of the present invention controls current, using the currents and voltages of the U-V-W coordinates as they are. Besides, it takes into consideration the influence of the sampling time on the discrete-time control. As a result, when the sampling time is fixed, a current control system providing quicker response can be realized .